1. Field of the Invention
The present invention relates to a solid-state imaging device represented by a complementary metal-oxide semiconductor (CMOS) image sensor and a camera system.
2. Description of Related Art
In recent years, the CMOS image sensor attracts attention as a solid-state imaging device (image sensor) which may replace charge-coupled device (CCD).
This is because the CMOS image sensor solves problems that a system is complicated considerably since a dedicated process is necessary for the manufacture of a CCD pixel, a plurality of power supply voltages are necessary for the operation, and it is necessary to combine and operate a plurality of peripheral ICs.
The manufacture of CMOS image sensors can use the same manufacturing process as that of a general CMOS type integrated circuit. CMOS image sensors may be driven with a single power supply. Further, an analog circuit and a logic circuit using a CMOS process may coexist in the same chip, and therefore it has a plurality of significant advantages that the number of the peripherals ICs can be reduced.
An output circuit of CCD mainly provides one channel (ch) output using an FD amplifier which has a floating diffusion layer (FD: Floating Diffusion). On the other hand, the CMOS image sensor includes the FD amplifier for each pixel, and the output is mainly of a column parallel output type in which a row in a pixel array is selected and pixels therein are simultaneously read out in a column direction. This is because it is difficult for the FD amplifier arranged in the pixel to obtain a sufficient drive capability, and therefore it is necessary to lower a data rate and parallel processing is considered to be advantageous.
In general, in the case of resetting the pixel in the CMOS image sensor, a method is often employed in which the pixels are reset one by one for each row (this method is hereafter referred to as “rolling shutter”). A rolling shutter operation will be described with reference to a particular circuit example.
FIG. 1 is a diagram showing an illustrative embodiment of the pixel of the CMOS image sensor composed of four transistors.
This pixel 1 includes a photoelectric conversion element 11 composed of a photo-diode, for example. Each one of the photoelectric conversion elements 11 includes four transistors as active elements, i.e., a transfer transistor 12, a reset transistor 13, an amplification transistor 14, and a selection transistor 15.
The photoelectric conversion element 11 performs photoelectric conversion of incident light to an electric charge (herein electron) corresponding to an amount of the light.
The transfer transistor 12 is connected between the photoelectric conversion element 11 and the floating diffusion FD. By supplying a drive signal to the gate (transfer gate) through a transfer control line LTx, the electron resulting from the photoelectric conversion by the photoelectric conversion element 11 is transferred to the floating diffusion FD.
The reset transistor 13 is connected between a power supply line LVDD and the floating diffusion FD. By supplying a reset signal to the gate through a reset control line LRST, a potential of the floating diffusion FD is reset to a potential of the power supply line LVDD.
A gate of the amplification transistor 14 is connected to the floating diffusion FD. The amplification transistor 14 is connected to an output signal line 16 via the selection transistor 15. The amplification transistor 14 and a constant current supply outside pixel array constitute a source follower.
When an addressing signal (selection signal) is supplied to a gate of the selection transistor 15 via a selection control line LSEL to turn the selection transistor 15 on, the amplification transistor 14 amplifies the potential of the floating diffusion FD, and outputs a voltage according to the potential to the output signal line 16. The voltage outputted from each pixel via the output signal line 16 is outputted to a column circuit (column processing circuit).
A reset operation of this pixel is such that the transfer transistor 12 is turned on to transfer the electric charge accumulated in the photoelectric conversion element 11 to the floating diffusion FD, so that the electric charge accumulated in the photoelectric conversion element 11 is discharged.
In this case, the floating diffusion FD turns on the reset transistor 13 to discard the electric charge to the power supply side in advance so that the electric charge of the photoelectric conversion element 11 can be received in advance. Alternatively, while the transfer transistor 12 is turned on, the reset transistor 13 may be turned on simultaneously to discard the electric charge directly to the power supply.
The series of the operations may be simply referred to as “pixel reset operation”.
On the other hand, in a readout operation, the reset transistor 13 is first turned on to reset the floating diffusion FD, and outputs it to the output signal line 16 via the selection transistor 15 which is turned on in the state. This is referred to as P phase output.
Next, the transfer transistor 12 is turned on to transfer the electric charge accumulated in the photoelectric conversion element 11 to the floating diffusion FD, and the output is outputted to the output signal line 16. This is referred to as D phase output.
A difference between the D phase output and the P phase output is obtained outside the pixel circuit and considered as an image signal by cancelling reset noises of the floating diffusion FD.
For the sake of brevity, the series of operations may be simply referred to as “pixel readout operation”.
FIG. 2 is a diagram showing an illustrative embodiment of a general structure of the CMOS image sensor (solid-state imaging device) in which the pixels of FIG. 1 are arranged in the shape of a 2-dimensional array.
A CMOS image sensor 20 of FIG. 2 includes a pixel array unit 21 in which the pixel circuits as shown in FIG. 1 are disposed in a two-dimensional array, a pixel drive circuit (vertical drive circuit) 22, and a column circuit (column processing circuit) 23.
The pixel drive circuit 22 controls the turn-on/turn-off of the transfer transistors 12, the reset transistors 13, and the selection transistors 15 of the pixels of each row.
The column circuit 23 receives data of a pixel row readout-controlled by the pixel drive circuit 22, and transfers the data to the latter signal processing circuit.
FIG. 3 is a chart showing a timing chart of the rolling shutter operation of the circuit as shown in FIG. 2.
As shown in FIG. 3, the pixel reset operation is performed in turn for each row, following which the pixel readout operation is performed in turn for each row.
The pixel of each row stores a signal in the photoelectric conversion element during the pixel reset operation and pixel readout operation, and the signal is read out by the pixel readout operation.